The Laser

The word laser stands for “Light Amplification by Stimulated Emission of Radiation”. The laser is a device that produces an intense beam of coherent monochromatic and unidirectional light as a result of co-operative emission from many atoms. This light beam may be intense enough to vaporize the hardest and most heat-resistant materials.

Principle of Laser:

Consider an assembly of atoms of some kind that have metastable states of excitation energy hv. Suppose we somehow raise a majority of the atoms to the metastable level. If we now shine light of frequency (v) on the assembly, there will be more induced emission from the metastable level than the induced absorption by the lower level. Thus, the result will be an amplification of  the original light. This is the concept that underlies the operation of the laser.

Population Inversion:

Under ordinary conditions of thermal equilibrium, the number of atoms in the higher energy state is considerably smaller than the number in the lower energy state (Since, by the Boltzmann law, N2/N1 = e^{(E2-E1)/kT} i.e., N2<N1. Hence there is very little stimulated emission compared with absorption. Let, by some means, the atoms be initially excited so that there are more atoms in the higher energy state E2 than in the lower energy state E1. Then we have N2>N1. This is known as Population Inversion.

Pumping:

The method of producing population inversion is called pumping. One type of pumping is "optical pumping". Consider a material whose atoms can reside in three different states as shown in figure (a). Atoms in ground state are pumped to state E3 by photons of energy hv' = E3-E1. The excited atoms then undergo non-radiative transitions with a transfer of energy to the lattice thermal motion, to the level E2. They remain in this metastable energy state for a comparatively  long time. Thus there will be more atoms in the higher metastable energy state E2 than in the ground state E1, i.e., we have a "population invertion", atoms in the metastable state E2 are now bombarded by photons of energy hv = E2-E1, resulting in a stimulated emission giving an intense, coherent beam in the direction of the incident photons. This is the method used in the ruby laser.

Mainly, Laser are of three types:

a. Ruby Laser

b. Helium-neon Laser

c. Semiconductor Laser

 

a. Ruby Laser:

It consists of a ruby cylindrical rod whose ends are optically flat and accurately parallel (figure c). One end is fully silvered and the other is only partially silvered. The rod is surrounded by a glass tube. The glass tube is surrounded by a helical xenon flash tube which acts as the optical pumping system.

The energy level diagram of these chromium ions is shown in above figure (c). The Cr ions are excited from level E1 to level E3 by the absorption of light of wavelength 550 nm from the xenon flash tube. The excited ions quickly undergo non-radiative  transitions with a transfer of energy to the lattice thermal motion, to the level E2. The E2 level is a metastable state with a lifetime of about 3 * 10^-3 (usual atomic life times are nearly equal to 10^-8 s). Now, the population of E2 level becomes greater than that of the E1 level. Thus “population inversion” is achieved.
Some photons are produced by spontaneous transition from E2 to E1, and have a wavelength of 694.3 nm (ruby red). The ends of the ruby rod act as reflecting mirror. Therefore, photons that are not moving parallel to the ruby rod escape from the side, but those moving parallel to it are reflected back and forth. These stimulate the emission of similar other photons. The chain reaction quickly develops a beam of photons all moving parallel to the rod, which is monochromatic and is coherent. When the beam develops sufficient intensity, it emerges through the partially silvered end.

Once all the chromium ions in the metastable level have returned to the ground level, the laser action stops. It is necessary to send one more flash of pumping radiation through the rod. Thus the ruby laser operates only in pulses.

b. Helium-neon Laser:

He-ne laser is a type of laser which consists He  and Ne in the ratio of 5:1 and also work with  the principle of laser operation.

Construction of He –Ne laser:

It consists of laser tube of approximately 5 mm in diameter and 0.5m long .It contains a mixture of helium and neon in the ratio of 5:1 at a total pressure of about 1 torr. The tube has parallel mirrors, one of them is partly transparent at both ends. The spacing of the mirrors is equal to an integral number of half wavelengths of laser light. The tube contains two electrodes which are connected to high voltage power supply so that electric field is set up in the tube. The mixture of the gas is ionized by passing electric current through it.

Working:

When the power supply is switched on, the electric field is setup in the tube. Helium atoms are excited very efficiently by electron impact into the 2s level as in the figure while the neon atoms are much readily excited by the electrons. The excited 2s state of helium is relatively long lived. The energy of this level(20.61 eV) is almost the same as the energy of the 5s level in neon (20.61 eV). Hence the energy of the helium atoms is easily transferred to the neon atoms when they collide. This preferential transfer of the neon atoms to the 5s state results in a population inversion between the 5s and 3p states. The purpose of the He atoms is thus to help achieve a population inversion in the Ne atoms. The spontaneous transitions from the 5s state to the 3p state, produce photons of wavelength 632.8 nm, which then trigger stimulated transitions. Photons travelling parallel to the tube are reflected back and forth between the mirrors placed at the ends, and rapidly build up into an intense beam which escapes through the end with the lower reflectivity. The Brewester end windows allow the light of one polarization to pass through without any reflection losses. Because the electron impacts that excite He and Ne atoms occur all the time. Thus, He-Ne laser operates continuously.

c. Semiconductor Laser:

Semiconductor Laser are those laser which are compact, efficient and can be fabricated with ease, however their monochromaticity, coherence and directionality are inferior to those of other lasers.   

Properties of a Laser Beam:

a.    The light is very nearly monochromatic.

b.    A laser beam diverses hardly at all.

c.    The light  is coherent with the waves all exactly in phase with one another.

d.    The beam is extremely intense.

Applications of Laser:

The laser is used in holography, radio communication in outer space, piercing holes in metals, detecting and ranging objects at great distances, welding, surveying  and delicate surgery.